Piezoelectrically driven ultrasonic generator

ABSTRACT

Ultrasonic system for measuring fluid flow by comparing the responses from pulsed beams transmitted obliquely in opposite directions through fluid flowing in a pipe section. The transducer assembly includes a driving element comprising a piezoelectric generator in the form of a disk with a high mass backing element bonded to one face and an acoustic wave transformer bonded to the other face. The wave transformer element varies in cross-section in an axial direction, comprising disks of maximum dimension at the generator face and at the radiating face, which are separated by a connecting portion of minimum dimension. In preferred form, the wave transformer is shaped like a barbell. An advantage of this configuration is that the beam pattern is a function of frequency, having a dominant central lobe at about 45 kilocycles per second which becomes relatively depressed with respect to increasing side lobes as the frequency is increased. Although the invention has been described in general terms, the primary uses of the system and components as disclosed herein are for the measurement of the flow of gases.

United States Patent Scarpa 1 PIEZOELECTRICALLY DRIVEN ULTRASONICGENERATOR [75] Inventor: Thomas J. Scarpa, Edison, NJ.

[73] Assignee: Scarpa Laboratories, Inc.,

Metuchen, NJ.

[22] Filed: Sept. 4, 1973 [21] Appl. No.: 394,407

[52] US. Cl 310/8.2; 73/194 E; 73/715 US; 116/137 A; 116/D1G. 19;310/8.3; 310/8.7; 310/9.1; 340/8 MM; 310/9.7; 310/26 [51] Int. Cl H04r17/00 I58] Field of Search 2110/26, 8.2, 8.3, 8.7, 310/9.1, 9.4; 73/675R, 67.6, 67.8, 69, 71.5 (US. only). 194 A, 194 B, 194 E; 340/8 MM;259/1; 116/137 A, D16. 19; 181/5 1, .5 AG; 51/59 SS [56] ReferencesCited UNITED STATES PATENTS 3,072,808 1/1963 Plesset et ul. 310/263,140,859 7/1964 Scarpa 3l0/8.2 X 3,210,580 10/1965 Bodinc, Jr 310/833.210.724 10/1965 Jones ct a1. 310/26 X 3,283,182 11/1966 Jones ct a1.3l0/9.7 X 3,341,935 9/1967 Ba|amuth....... 310/26 X 3,578,996 5/1971Balamuth 3111/26 X 1 June 24, 1975 Primary E.raminer-Mark O. BuddAttorney, Agent, or FirmMartha G. Pugh, Esq.

i 5 7 1 ABSTRACT Ultrasonic system for measuring fluid flow by comparingthe responses from pulsed beams transmitted obliquely in oppositedirections through fluid flowing in a pipe section. The transducerassembly includes a driving element comprising a piezoelectric generatorin the form of a disk with a high mass backing element bonded to oneface and an acoustic wave transformer bonded to the other face. The wavetransformer ele ment varies in cross-section in an axial direction,comprising disks of maximum dimension at the generator face and at theradiating face, which are separated by a connecting portion of minimumdimension. ln preferred form, the wave transformer is shaped like abarbell. An advantage of this configuration is that the beam pattern isa function of frequency, having a dominant central lobe at about 45kilocycles per second which becomes relatively depressed with respect toincreasing side lobes as the frequency is increased. Although theinvention has been described in general terms, the primary uses of thesystem and components as disclosed herein are for the measurement of theflow of gases.

8 Claims, 15 Drawing Figures SHEET PATENTEDJUN24 I975 Pmmmm m5 I Il-AMPLIFIER OSCILLATOR OSCILLATOR AMPLIFIER I FREQUENCY METER INDICATORmvmea'cmcun 57 1 PIEZOELECTRICALLY DRIVEN ULTRASONIC GENERATORBACKGROUND OF THE INVENTION This relates in general to ultrasonicflowmeter systems and, more specifically, to the transducer assembliesemployed in such systems, particularly for measurement of the flow ofgas.

In ultrasonic flowmeter systems of a type disclosed in the prior art,the velocity of fluid flowing in a test pipe section is measured bytransmitting a pulsed ultrasonic beam obliquely across the pipe sectionfrom a transmitting transducer interposed at one point in the pipe to areceiving transducer interposed at a second position in the pipe, spacedup or downstream from the first. When an ultrasonic wave is transmittedthrough the test fluid, the time required for such transmission isdependent on the velocity of the ultrasound in the fluid, the length ofthe path over which the beam is transmitted and the velocity at whichthe fluid is flowing. The velocity of fluid flow either adds to orsubtracts from the transmission time, depending on whether the beam istransmitted against or in the direction of the stream of flow. Thus, afeedback circuit produces a signal having a frequency of repetitionwhich varies in accordance with the time required for the ultrasoundwaves to travel over a predetermined path in the fluid.

Such ultrasonic systems are well-known in the art, as disclosed, forexample in US. Pat. No. 2,669,121 to R. L. Garman et al., issued Feb.16, I954, and by others.

In accordance with prior art practice, the transducers used fortransmitting and/or receiving the ultrasonic beam in such a system wereso constructed that the maximum lobe of the emitted beam was directed insubstantially a normal direction from the face of the transducer. Thus,in order to provide a beam capable of crossing the stream of fluidflowing in the pipe at an oblique angle, it was necessary to build ashoulder or supporting nipple onto the surface of the pipe. Thisobviously requires precise modifications in the pipes to which such asystem is applied. Moreover, it creates a series of recesses in theinner pipe wall, associated with the obliquely positioned transducerswhich tend to cause interruptions or turbulence in the fluid flow beingmeasured.

In addition, particularly in measurement of the flow rates of gases orlow density liquids, problems arise in impedance matching the radiatingmember of the transducer assembly to the fluid in such a way as to avoidunwanted reflections and thereby energy loss.

SUMMARY OF THE INVENTION Accordingly, it is a principal object of thepresent invention to provide an ultrasonic system for measuring fluidflow in pipe systems which is substantially more sensitive, moreefficient and more economically con structed than the systems of theprior art.

A particular object of the present invention is to provide an ultrasonicsystem for measuring fluid flow in pipes transporting gases and lowdensity fluids in which the ultrasonic transducing assemblies areimpedance matched to the measured fluid.

Another object of the invention is to provide a system in whichalterations in the test pipe surface for installing transducers areminimized.

0 the radiating face may be controlled as a function of frequency.

These and other objects, features and advantages will be apparent tothose skilled in the art as set forth in the invention hereinafter.

The present invention relates to ultrasonic systems for measuring fluidflow in a pipe in which one or more pulsed beams is transmittedobliquely through the fluid from a transmitting transducer interposedthrough the pipe wall in one position to a receiving transducerinterposed through the pipe wall in another position, the effect of thefluid flow velocity on the pulse transmission time being computedelectronically.

in order to facilitate the operation of such a system, the presentinvention contemplates a novel type of transducer assembly whichincludes a driving element comprising a longitudinal thickness-modepiezoelectric generator in the form of a disk having a backing ele mentof relatively high mass bonded to one face and an acoustic wavetransformer bonded to the other face. The whole assembly is preferablyan integral number of quarter-wavelengths along the principal axis inthe vibrating frequency of the system. The transformer diniinishes incross-section from a maximum at the bond with the driving element to arelatively small crosssectional dimension along its axis at a parallelplane remote from the bond. The small cross-sectional dimension isbroadened out at the radiating end to form a center driven radiatingdisk of relatively small mass. The diameter of the radiating disk ispreferably a halfwavelength in the operating frequency; and, it isdisposed parallel to the face of the piezoelectric generator and normalto the axis of the transformer. The mass ratio between the backingelement, which may comprise a metal cylinder, and the transformingelement lie anywhere within the range 2:1 to 30:1, thereby producing aparticle velocity transformation of from 2:1 to between the face of thepiezoelectric generator and the radiating disk. The transformer,including the disk, functions to step down the impedance by increasingthe area of contact at the radiating surface, which moves in small arcvibrations at high velocity. Thus, the low impedance at the face of theradiating disk is substantially equal to that of air, or any other lowdensity fluid. This has the advantage of greatly increasing theefiiciency of the transducer assembly by reducing the energy losses fromdistortions and reflections at the interface. A tight beam of smallangular divergence is radiated by transducer assemblies designed inaccordance with the present invention. This permits the ultrasonic beamsso radiated to be transmitted and received over greater distances thanwith the same expenditure of energy using prior art structures.

In accordance with a preferred form of the invention disclosed herein,the solid energy transforming member may assume a shape somewhat like abarbell with disks of equal diameter on the two ends connected by acylindrical axial portion having a diameter about onethird that of theend disks. In the disclosed embodiment, the left-hand disk which isbonded to the face of the piezoelectric generator is substantiallythicker than the radiating disk.

In accordance with a variation, the cross-sectional dimension of thetransforming member decreases exponentially until it reaches a minimumcross section of, say, one-tenth the diameter bonded to thepiezoelectric element. The minimum cross-sectional plane is then bondedor otherwise integrally connected in driving relation to the center of athin disk about the same diameter as the piezoelectric generatingelement. There are numerous other suitable shapes for the transformingelement of the present invention, the only requirement being that thecross-sectional dimension decrease in a manner symmetrical about thelongitudinal axis from a maximum at the area of contact with thepiezoelectric generator surface to a minimum along the axis at a planeremoved from the generator surface; and, that the minimumcross-sectional area function as the central driving means for anexpanded radiating surface comprising a relatively thin disk normal tothe axis. Such a transformer configuration bonded to one face of apiezoelectric generator, to the other face of which is bonded a backingelement of relatively higher mass, constitutes a transducer assemblywith numerous advantages. It produces a highly efficient narrowultrasonic beam impedance matched to gas and low density liquids.

As a further feature of the present invention, the transducer assemblydescribed in the foregoing paragraphs is supported in a coaxialacoustically isolating housing for providing electrical connectionsbetween the energizing circuit and the piezoelectrically activetransducer, which may comprise a sandwich transducer of the bimorphtype.

A particular advantage of the structure of the present invention is thatthe beam pattern has been found to be a function of frequency. Whereasat a frequency of about 45 kilocycles per second, the beam has adominant central lobe and relatively suppressed side lobes, I have foundthat by increasing the operating frequency to about 90 kilocycles persecond, the central lobe is depressed and the side lobes becomedominant.

When transducers of the type disclosed herein are incorporated in anultrasonic flowmeter of the general circuit arrangement disclosed byGarman et al., supra, substantial advantages ensue in that such aflowmeter is especially adapted to measure the flow rates of gas andother low density fluids with high efficiency. Moreover, assuming thetransducer assemblies are operated in a frequency range producingdominant side lobes, it becomes unnecessary to build shoulders ornipples into the pipe wall for respectively supporting the individualtransducer assemblies in an oblique direction with reference to the axisof the pipe. Instead, the transmitting and/or receiving transducerassemblies may be interposed into the test pipe section in directionsrespectively normal to the pipe surface. In this type of operation thepulsed side lobe beams are transmitted and/or received obliquely in thepipe from two or more identically operated transducers positioned inobliquely spaced-apart positions so that while one transducer assemblytransmits an obliquely directed beam, another transmitter assembly ispositioned to receive the obliquely transmitted beam, and vice versa.

Other objects, features and advantages will be under stood from adetailed stun or the specification hereinafter with reference to thedrawings of which the following is a short description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall perspectiveshowing of the narrow beam transducer assembly of the present inventron;

FIG. 2A is a longitudinal section through the plane indicated by thearrows 2-2 of FIG. 1;

FIG. 2B is a modification of a section of the left of the plane x-x inFIG. 2A, including a feedback crystal;

FIG. 3A is an enlarged sectional showing of the backing element 3 ofFIGS. I and 2;

FIG. 3B is an end elevation showing the face 3b;

FIG. 4 is an enlarged side elevational showing of the backing element 3of FIG. 1, showing the recessed area for the screw hole 3d;

FIG. 5A is an enlarged side elevational showing of the acousticaltransformer 2 of FIG. 1;

FIG. 5B is an end elevational showing of the left-hand end face 2a ofthe acoustic transformer 2 of FIG. 5A;

FIG. 6 is a showing in perspective of the transducer assembly in oneform of housing for incorporation into a test pipe system;

FIG. 7 is a longitudinal section of the assemblage of FIG. 6;

FIG. 8 is a schematic cross-sectional showing of the transducer assemblydisclosed in FIGS. 1 and 2, indicating the general outline of theultrasonic beam radiated at an operating frequency of about 45kilocycles per second, comprising an enlarged central lobe and minimalside lobes;

FIG. 9 is a schematic cross-sectional showing of the transducer assemblydisclosed in FIGS. I and 2, indicating the general outline of theultrasonic beam radiated at an operating frequency of about kilocyclesper second, comprising enlarged side lobes, the central lobe beingsubstantially suppressed;

FIG. 10 is a schematic circuit showing of an ultrasonic flowmeter of thepulsed beam type employing a rectangular array of four transducerassemblies of a type shown in FIG. 8, driven at about 45 kilocycles persecond, and each characterized by a beam having a prominent centrallobe;

FIG. 11 is a schematic circuit showing of an ultrasonic flowmeter of thepulsed beam type employing a rectangular array of four transducerassemblies of a type shown in FIG. 9, driven at about 90 kilocycles persecond, and each characterized by a beam having a pair of enlarged sidelobes and a suppressed central lobe; and

FIG. 12 is a schematic circuit showing of an ultrasonic flowmeter of analternative from to that indicated in FIG. 11 employing a triangulararray of three trans ducer assemblies of a type shown in FIG. 9, drivenat about 90 kilocycles per second, and each characterized by a beamhaving a pair of enlarged side lobes and a suppressed central lobe.Detailed Description Reference will now be made to FIGS. 1 and 2A of thedrawings which show a preferred from of the transducer assembly of thepresent invention.

The radiating face 2b of the transducer assembly may be considered to bea center driven thin plate, vibrating in a flexural mode. The drivingforce is a longitudinal or compressional wave that is converted by thegeometry into a flexural, radial wave in the disk. In such a transition,the phenomenon of velocity dispersion occurs which makes the resonantfrequency of the disk difficult to mathematically predict and is usuallyarrived at by empirical experimentation.

The velocity constant c, for the flexural wave in the disk 2b maycoarsely be computed from an equation that is derived from a fourthorder differential equation given by P. M. Morse, Vibration and Sound,second edition, McGraw Hill, New York, i948, pp. 154 and 209. For aplate whose width a: c;= [Y K /p(l 6 /4 (2 DIM where:

c;= Flexural velocity Y0 Youngs Modulus a Plate thickness, of 2b asindicated in FIG. 2

K Radius of gyration (for a plate a/ m p Density of the plate 6 PoissonsRatio f Frequency of vibrations.

The combination 1, disclosed in H0. 1, in perspective, has two visibleelements, namely, the acoustic wave radiator 2 and the backing element3. As shown in the sectional view of FIG. 2A, a composite piezoelectricdriving unit of the bimorph type comprising elements 4a ,4b is mountedcontiguously with the circular inner faces of the backing element 3 andthe acoustic wave radiator 2, or transforming element.

The active elements 4a,4b may be any piezoelectric crystalline bodyknown in the art, formed or polarized to vibrate principally in aresonant-thickness mode of vibration. For the purposes of the presentinvention, elements 4a,4b are preferably disk-shaped wafers oflead-zirconate-titanate dimensioned in accordance with the desiredfrequency. In the present illustrative embodiment, they are 0.500 inchin diameter and, say, 0.078 inch thick; and are constructed to vibratein a longitudinal-thickness mode of a resonant frequency of about onemegahertz. It will be understood that the wafers 4a,4b are thicker orthinner, depending on the desired resonant frequency of the principalmode. Disks of lead-zirconate, titanate, of a type and dimensionsuitable for the uses of the present invention, may be obtainedcommercially in various diameters and in thicknesses of 0.5 inch orless.

As a step in their preparation, the piezoelectric wafers 4a,4b arepolarized in accordance with well-known prior art practice by theapplication of a polarizing voltage in a thickness directionsubstantially normal to the major faces of the wafer, while thetemperature is lowered through the Curie temperature. Moreover, theelements 4a,4b are aged to give them stable electroacousticcharacteristics, poling and aging having been carried out by thesuppliers in a manner taught, for example, in a bulletin entitled Agingof Ferroelectrics," New Jersey Ceramic Research Station, RutgersUniversity, New Brunswick, Technical Report No.1, Rt. 1, Til-59428 (JulyI, 1959) (AD-227-737-l004).

An important step in the process of preparing these piezoelectricelements is the careful cleansing and preparation of the contactingsurfaces prior to assemblage into any of the combinations disclosedhereinafter. For example, the wafers 4a,4b when obtained commercially,or after prepolarizing treatment, may have permanent electrode coatingson their major parallel surfaces. For some embodiments of the presentinvention these are carefully removed by lapping to a depth of 0.005inch with any of the lapping COlTPOUfiJh known in the art, such as, forexample, aluminum oxide and water.

After the foregoing treatment the surfaces of the ceramic wafers 4a,4bare further cleaned by exposing the elements to ultrasonic cleaningaction in a container of, for example, an isoprophyl alcohol or acetone,or any similar solvent which is characterized by rapid evaporation, andwhich is not readily absorbed by the ceramic. The ultrasonic vibrationsare generated in the cleansing liquid by means of, for example, a I00 to200 watt generator operation at a frequency of, say, 40 kilocycles persecond, for about l0 minutes.

In addition to the use of lead zirconate-titanate, which is thepiezoelectric disclosed in the present illustrative example, any otherceramic or piezoelectric crystalline materials may be employed for thepurposes of the present invention which are capable of producing anelement which vibrates in the longitudinalthickness mode, such as, forexample, barium titanate, x-cut quartz, etc.

As pointed out hereinbefore, the crystal elements 40,412 are sandwichedtogether with an electrode coating, preferably an acoustic conductingepoxy characterized by an electrical resistivity of at least about 0.01ohmcentimeter at 25 C., providing a coating which forms the innerelectrode 5. A suitable conductive cement for this purpose can beobtained from the Hysol company as No. 4238, which is cured with Hysolhardener No. 3469, or a similar combination. The external electrodes6a,6b comprises similar coating applied to the external surfaces ofelements 4a,4b. As previously stated, the piezoelectric sandwich 4a,4bis interposed between the cylindrical backing element 3 and theacoustical transforming element 2, which will now be described.

Although the piezoelectric element 4a,4b is disclosed herewith as abimorph, it will be understood that a single crystal wafer can be used.Moreover, although elements 4a,4b are disclosed as being cylindrical inthe present illustrative example, it will be understood that they can beother shapes, such as rectangular.

Referring to the FIGS. 3A, 3B and 4, it will be seen that in the presentexample the backing element 3 comprises a substantially cylindricalmetal element of stainless steel having an outer diameter of 0.625 inchand an axial length of 0.410 inch. The rightand left-hand faces 30 and3b each have circular recesses symmetrical about the axis which are 0.02inch deep and have a diameter of 0.500 inch. The inside faces of each ofthese recesses are respectively grooved with a series of annular grooves0.010 inch deep and spaced apart 0.050 inch. The recess of right-handface 3b serves to accommodate the face of the piezoelectric element 4a,together with the cementing epoxy mixed with conducting coating which isforced into the indentations to provide a better bond. Adjacent theleft-hand face is a lengthwise indentation 3c about 0.250 inch wide and0.360 inch deep which is rounded at the inner end to accommodate a screwhole 3d which is 4/40 inches in diameter with a tap one-fourth inchdeep. This serves for mounting the backing element 3 when the transducer has been assembled. It will be understood that in addition tostainless steel, as disclosed for the present illustrative example,backing element 3 may alternatively be formed of any high densityelement, such as high carbon steel, monel metal, or even ceramicmaterial having sufi'rcient density to serve as a loading element.Although the backing element 3 is disclosed herein as being cylindrical,it will be understood it is not limited to that shape.

In addition to the piezoelectric elements 4a,4b, a modificaton of theinvention shown in FIG. 28 includes an auxiliary piezoelectric element 7for feedback purposes, which may be of any thickness between, forexample, 0.15 inch and 0.78 inch. This is bonded into the recess of theleft-handed surface of the backing element 3 in the same manner that thecomposite element 4a,4b is bonded to the right-hand recess. Similarly,electrode coatings 8 and 9 are included which may also compriseconducting epoxy of the type previously described.

The unique element of the combination disclosed is the energytransforming and acoustic wave radiating element 2. This combination ispreferably formed of a metal such as aluminum, titanium, monel orstainless steel.

In accordance with the present invention, the element 2 hassubstantially smaller mass than the element 3. However, the ratio of themass of the backing element 3 to the acoustic wave tranforming-radiatingelement 2 may lie anywhere within the range 1:1 up to 30:1 or 40:1 orhigher.

The function of the impedance member 2 is two-fold. One function is toprovide a substantial increase of the particle velocity from its drivingend 20 to its driven and 2b. Another function is to provide a driven endhaving an impedance substantially matched to air or gas impedance, whichis center-driven and has a large radiating surface constructed towithstand high temperatures and pressures, and substantially resistantto corrosion, This is accomplished by providing an element shaped, forexample, as shown in FIG. A, in which the left-hand, or driven, portioncomprises a disk-like end 20 which has a diameter 0.750 inch and iscylindrical having a thickness in an axial direction of 0.135 inch. Asexplained with reference to the element 3, the lefthand face of thedisk-like end 20 has a recess 10 matching the recess in the right-handend of the cylindrical element 3 for mounting the face 4b of thepiezoelectric sandwich element 4a,4b. As described with reference to thebacking element 3, the recessed portion of the left-hand face isprovided with a series of annular grooves ll which are 0.010 inch deepand spaced apart 0.050 inch for receiving the conducting epoxy coatingon the contacting face of the piezoelectric sandwich element 4b. In thepresent embodiment, the transforming member 2 is 062510.005 inch long,in an axial direction, tapering conically to a diameter at mid-sectionof about 0.250 inch. The right-hand end is expanded in cross section toa diameter of 0.750 inch, forming a plate-like radiating member 2b 0.090inch thick, the radiating face being made parallel within :51002 inch tothe face of the driven end 20 which is in contact with the electrodecoating on the face of piezoelectric active element 4b. The diameter ofthe radiating element 2b preferably approximates a half-wavelength inthe vibrating frequency of the transducer; altough it will be apparentthat this is only approximate, as the system is designed to operate overa range of frequencies. In order to have a pronounced main lobe thediameter of the disk should be limited to under one wavelength. However,the smaller the diameer, the less is the impedance match. If thediameter exceeds one wavelength, the vibration of the disk breaks upinto multiple modes and produces a suppressed main lobe and pronouncedside lobes, generally two main lobes with a multiplicity of minor lobes.The axial length of the combination, as shown in FIG. 2A, from thecenter of the radiating end 2b to the left-hand face of the backingelement 3, preferably approximate an integral number ofquarter-wavelengths in the vibrating frequency.

It will be understood that although a specific embodiment of thetransforming element 2 is disclosed herein by way of illustativeexample, the transforming element can take other forms within theteaching of the present invention, in which the driving means applied atthe center of the right-hand radiating element 217 serves to step-up theparticle velocity anywhere within the range 2:1 to :1; and, the diskelement 2b, which is disposed at the high velocity end of thetransformer, serves to substantially step-down the impedance between thetransforming element and the contacting medium by substantiallyincreasing the area of contact.

It will be apparent that the manner in which the cross section of thetransforming means 2 is reduced between the elements 2a,2b may assumethe form of any one of a number of cross-sectional curves well-known inthe art, such as cone, parabola, catenary, etc., as long as thecross-sectional dimension is tapered from a large diameter at thedriving end to a substantially small diameter at the plane at which theradiating member 2b is applied, so as to provide a velocity step-up.

Referring now to FIGS. 6 and 7 of the drawings, there is shown apreferred type of housing including a pipe fitting for a transducerassembly of the type described with reference to FIGS. 1 through 5B ofthe drawings.

P16. 6 is an external perspective showing of the transforming assemblyin the housing; whereas, FIG. 7 shows the complete assembly inlongitudinal section.

The transducer assembly comprising the active piezoelectric sandwichelement 4a,4b, backed by cylinder 3 and disposed in driving relation toa solid acoustic transformer 2, is enclosed in the stainless steeltubular housings 12 and 13. The latter are fitted together inlongitudinal relation with the right-hand end of tube 12 bonded into aslight recess in the left-hand end of tube 13. The latter is 0.895inches in outer diameter and 0.748 inches in inner diametr, beingslightly recessed at the left-hand end, as indicated. Tube 13 is fittedcoaxially around the solid transformer element 2 so that the right-handface 2b of the latter protrudes about 0.015 inches from the end of thetube in an axial direction. Tubular housing 12 is 0.850 inch in outerdiameters, 0.748 inches in inner diameter and 1.187 inches in axiallength. Screwed onto the left-hand end of the tubular housing 12 is anend plug 12b having an annular flange 12c which is one-fourth inchthick, 1.020 inch in outer diameter. At its right-hand end, plug 12b isreduced in diameter to form a screw-threaded nipple about 0.733 inch inouter diameter which protrudes one-fourth inch coaxially for internalengagement with a matching screw-threaded recess in the inner end of thetube 12. An axial bore 12a which is 0.078 inch in diameter, foraccommodating lead wire 16, extends through the flange 12b and thenipple.

Flange 12c provides an annular shoulder extending outwardly about 0.075inches in a radial direction from the screw-threaded surface of thenipple. The flange includes an annular notch 12d which is 0.068 incheswide and 0.035 inch deep, symmetrically disposed between the oppositefaces of the flange. The

lefthand face of the plug 12b also includes an annular o-ring groovehaving a means diameter 0.95 inch. which is 0.04 inch wide and 0.015inch deep. An annular collar 21 which is 0.065 inch in inner diameterand 0.0884 inch in outer diameter one-fourth inch in axial length. andexternally screw-threaded. is fitted near the left-hand end of tube 12,providing an annular clear ance 0.05 inch wide with the shoulder offlange 12c. The aforesaid three annular notches are constructed toretain three o-rings 22, 23 and 24. which serve to acoustically isolatedthe inner transducer assembly and its housing from tubular brassenclosure 14, in a manner which will now be described.

The tubular enclosure 14 comprises an enlarged open right-hand end 140.externally screw-threaded at 1 /2 turns per inch. It is 1%inch inmaximum outer diameter. 0.680 inch in overall length and slightlytapered from left to right in accordance with the National Plumbing codetaper for one-inch pipe. The open end of enclosure 14 forms a cavity1.041 inch in diameter and 0.567 inch along the axis. terminating at itslefthand end in an annular inner shoulder 14c which is about one-fourthwide in a radial direction. The cavity is internally screw-threaded atits open right-hand end at a pitch of 32 turns per inch.

At its left-hand end. tubular enclosure 14 has a square tubularextension 14b, symmetrical about the axis. the outer dimension of whichis 0.70 inch on a side. and which extends externally three-eighth inchto the left of the flanged screwthreaded portion 14a. The diameter ofthe cylindrical inner cavity of enclosure 14 drops off from the innershoulder 146 for an axial length of about 0.1 13 inch, to form a smallercylindrical inner cavity 14f which is one-half inch in diameter and 0.4inch along the axis. This is partly closed at the left-hand end. exceptfor an axial opening 14g which is 0.209 inch in diameter to accommodatean Amphenol BNC-type connector. as will be described. In the presentexample, the axial depth of opening 14g is 0.055 inch; and. it leadsinto an annular recess 14h which is 0.351 inch in diameter and 0.045inch deep at the center of the rectangular left-hand face of the extension 14b. The latter is constructed to mate with an Amphenol-typeplug assembly 18, which will now be described.

The AmphenoV coaxial-type connector is of particularly the general formindicated in .l. V. Malek et al.. U.S. Pat. No. 3,054,981, issued Sept.18, 1962, or C. W. Concelman US. Pat. No. 3.103.548, issued Sept. i0.1963. It comprises a tubular outer conductor 18a of silver, or otherconducting material, three-eighth inch in outer diameter, one-third inchthick, which extends live-eighth inch in axial length. terminating atthe center of a rectangular plate 18b, eleven-sixteenth inch byeleven-sixteenth inch and three-sixteenth inch thick. This has screwholes to accommodate the screws 19 at its four corners. so that it matesadjacent the left-hand face of the rectangular extension 14b of theouter housing 14, in contact with gasket 20, about one thirty secondsinch thick. co prising rubber. Neoprene" or another similar elastomer. Apair of diametrically opposite protuberances [80 provide means forlocking shell 18:: in electrical connection with an external coaxiallead. It will be apparent to those skilled in the art that the specificdimensions of pipe 141) and pipe plug 18b will vary so that eachAmphenol type connector 18 is fitted to the pipe plug securely. Beforethe connector 10 is put in place, a quantity of silicone rubber isinternally into the opening 14g to secure the connector. After theconnector is in place. the silicone is allowed to cure 24 hours. afterwhich a test is made to determine that there is no air leakage aroundthe connector.

Amphenol connector 18 has a cental conductor 180'. about one-sixteenthinch in diameter. embedded in an annular insulating member 180.three-sixteenth inch in diameter. This combination is embeddedinternally in a solid conducting sheath 14g which makes solid integralcontact with the inside of shell 18a, which extends from a positionabout five-eighth inch inside the left hand end and continuing throughto the right-hand face of the rectangular connecting plate 18b. where itprotrudes about one-sixteenth inch beyond the surface of the plate,surrounding the insulating portion 18a, which protrudes an additionalone-sixteenth inch. surrounding the axial conductor 16 which isconnected to the high potential electrode coating 5 of piezoelectricelements 4a,4b to which it is soldered or otherwise electricallyconnected.

The flanged end 12c of tubular housing 12 fits into the cavity in thescrew-threaded outer connector so the left-hand face rests against theo-ring 22, one inch in outer diameter and one-sixteenth inch inthickness, resting against internal shoulder 14c. The o-ring 22 ispreferably rubber or a rubber substitute. such as Neoprene or any otherinsulating material having a similar coefficient of elasticity. Thesecond similar o-ring 23 is interposed in the annular notch 12d; and.the third similar o-ring 24 rests against the shoulder formed by theright-hand face of the flange 12c. As previously mentioned. the functionof these three gaskets is to acoustically isolate the external housingfrom the transducer assembly. The combination, including o-ring 22. 23and 24, is held in place by the annular collar 21, the rings beingscrew-fitted into the internal threads of the outer tubular housing 140.The left-hand end of collar 21 contains two or three peripheral notcheswhich are one-eighth inch wide and 0.09 inch deep. These aresymmetrically spaced around the circumference of the collar.

Assuming the acoustically isolating o-ring 22, 23 and 24 to benonconducting or insulating material, a conducting tab 9, aboutone-fourth inch wide and one-half inch long, is force-fit between theexternal surface of the cylindrical shell 12 and the internal surface ofthe screw-threaded collar 21, so as to make electrical contact betweenthe two. providing a conducting path for the electrode coatings 6a,6bwhich are connected together to make electrical contact with thetransformer face 20. and hence. internal surface of the shell 12.

As an alternative to using the conducting tab 9, the elastomer o-rings20, 22, 23 and 24 are formed of a mixture of elastomeric material withmetallic powder. having a resistivity of at least 0.01 ohm-centimeter at25 c. For example, a suitable conductive epoxy cement can be obtainedfrom the Hysol Company as No. 4238, which is cured with Hysol hardenerNo. 3469 or a similar combination.

Referring to FIG. 8 of the drawings. let us assume that a transducerassembly of the type previously described is connected to a conventionalhigh frequency oscillator operating at a frequency of the order of 45kilocycles per second. The ultrasonic radiation pattern of the plate 2b,indicated in FIGS. 1 and 2A. includes a centrally directed ultrasonicbeam having a major central lobe which is substantially symmetricalabout the 1 axis. Laboratory measurements show that at a distance of onefoot from the center of the plate 20, along the x axis, the beamintensity varies in a relative manner as illustrated.

If a receiving meter is moved in a plane perpendicular to the face ofthe radiator through a series of points of identical ultrasonicintensity, a plotting of these points indicates a pronounced centrallobe a, flanked by a pair of substantially symmetrically disposed sidelobes b and c, considerably weaker intensity. These patterns assumemeasurements made in air or another gas.

If, however, the frequency of the oscillator, operating at the samevoltage and power, is increased to about 95 kilocycles per second, theradiation pattern changes to the general form indicated in FIG. 9.Measurements at the same level of ultrasonic intensity in a planeperpendicular to the face of the radiator produce a series of readingswhich, when plotted, indicate a radiation pattern of the form shown inFIG. 9 of the drawings. As indicated, the central lobe dis nowsuppressed, and a relatively larger amount of energy is contained in theside lobes. It is apparent that the operation of the transducer assemblyat the 95 kilocycles per second frequency, in such a manner that itincludes the prominent side lobes as shown, has many applications nothitherto shown in the art. Two of these will be described with referenceto FIGS. 11 and 12 of the drawings.

First, however, a gas flowmeter circuit of FIG. will be describedwherein are employed four transducer assemblies of the type indicated inFIGS. 1 and 2A, which are operated at a pulse modulated frequency of theorder of 45 kilocycles per second to produce in each case a beam patternhaving a single prominent lobe of the type indicated in FIG. 8 of thedrawings. The operation of the circuit shown in FIG. 10 is substantiallysimilar to that described in US. Pat. No. 2,669,121 issued to R. L.Garman et al. on Feb. 16, 1954, entitled Supersonic Flowmeter. Theultrasonic flowmeter shown schematically in FIG. 10 is designed so thatthe velocity of fluid in a pipe section is determined directly, theindication obtained being independent of the velocity of sound in thefluid. Different fluids transmit sound at different velocities, thevelocity of travel through any particular fluid varying withtemperature. By providing a device whose measured output is independentof such a variable, the necessity for recalibration and the allowancefor temperature errors and other variables are avoided.

When an ultrasonic wave is transmitted through a fluid, the timerequired for such transmission is dependent on the velocity of sound inthe fluid, the length of the path over which the ultrasonic wave istransmitted and the velocity of the fluid itself, the latter eitheradding to or subtracting from the transmission time depending on whetherthe ultrasonic wave is transmitted against or with the stream of flow.

In the flowmeter shown in FIG. 10, the time required for transmission ofthe ultrasonic waves over a predetermined path in the measured fluid isdetermined by the use of a feedback circuit which produces a pulsedsignal whose repetition frequency is a function of the transmissiontime. This leaves two variables: namely, the velocity of the fluid to bedetermined and the velocity of sound through the fluid. In order toeliminate the velocity of the sound by cancelling it out from the finalindicating instrumentality, two feedback circuits, driven from a commonsource of power, are utilized. One circuit includes amplifier 42 andpulse modulated, high frequency alternating current oscillator 44 whichenergizes transmitting transducer assembly 34 to transmit an ultrasonicbeam to receiving transducer assembly 36, so that the beam obliquelyintersects the fluid path. The other, which includes amplifier 39 andpulse modulated. high frequency alternating current oscillator 31,energizes transmitting transducer assembly 33 to transmit an ultrasonicbeam to receiving transducer assembly 37, so that the beam obliquelyintersects the fluid path in the opposite direction. Each of thetransducer assemblies 34, 35, 36 and 37 is so operated that it directs aprincipal ultrasonic beam substantially normally to the radiatingsurface. Hence, the axes of transducer assemblies 34 and 35 arerespectively mounted in nipples 32a and 330 which are disposed obliquelyin the wall of pipe section 30. Transducer assemblies 36 and 37 aresimilarly mounted in obliquely disposed pipe nipples 33b and 321). Insuch an arrangement, each separate path will generate a signal ofadifferent repetition frequency. The frequency of the signal in one path,namely from transducer assemblies 34 to 36, will be greater than themeans by an amount which is proportional to the velocity of the fuid;and, the frequency of the signal in the other path, from transducerassemblies 35 to 37, will be less than the means by a like amount.Signals are derived from both paths and are impressed in opposingrelation on the mixer circuit 46. The differ ence signal from mixercircuit 46, varying only as the velocity of the fluid, is used toprovide an indication in meter 47, which is a function of the velocityof fluid flow in the pipe.

The foregoing will be more clearly understood by reference to thedescription in the aforementioned US. Pat. No. 2,669,121 to Garman etal., supra, which describes in detail the manner of operation of thecircuit of FIG. 10, with the exception of the transducer assemblies 34,35, 36 and 37, which are specifically of the form described in FIGS. 1,2A, et seq. of this application, operated to radiate a beam having adominant central lobe, in the manner of FIG. 8.

Assume for further example that transducer assemblies 34, 35, 36 and 37are replaced with transducer assemblies 34', 35', 36' and 37' which areoperated in the manner indicated in FIG. 9 of the drawings, at afrequency of the order of kilocycles per second; and further, assumethat the circuit to which they are connected is substantially of theconfiguration and operation shown and described hereinbefore withreference to FIG. 10.

Because of suppressed central lobe and the angular spread ofapproximately thirty degrees between the side lobes e andf(FIG. 9), itis no longer necessary to modify the surface of the pipe section toprovide special nipples 32a, 32b, 33a and 33b in which to mount thetransducer assemblies as indicated in FIG. 10. Instead, pipe section30', as shown in FIG. 11, is substantially unmodified, except for thedrilling of an opening in a direction normal to the pipe wall for thepositioning of each of transducer assemblies 34', 35', 36' and 37' intheir respective housings. The latter are screwed, or otherwise closelyfitted, to protrude in a normal direction through the wall of pipesection 30', so that the radiating face 2b of each of the transducerassemblies is substantially flush with the inner surface of the pipe.

Referring to FIG. 9, it will be seen that each of the aforementionedtransducer assemblies, when driven at a frequency of about 95 kilocyclesper second, generates a pair of dominant ultrasonic side lobes fortransmitting and/or receiving, which are directed obliquely, each at anangle 6/2 relative to the direction of fluid flow in the pipe, where 9is the angle between the dominant side lobes. At the radiating face ofeach transducer assembly, one lobe is directed upstream, and the otherdownstream. Thus, with this modification, the system shown in FIG. 11has its transducer assemblies 34', 35, 36' and 37' connected insubstantially the same manner as the corresponding transducer assembliesof FIG. 10, in a pulse transmission and receiving system constructed andoperated in a substantially similar manner thereto.

It will be appreciated that alternative arrangements of the flowmetersystems of FIGS. 10 and 11 can be made using, for example, twotransducer assemblies positioned as indicated by 34 and 36', instead forfour, with the circuit modified so that the direction of wavetransmission is reversed at alternate intervals by alternately switchingeach transducer between the transmitting and receiving circuit.

Another alternative flowmeter system, making use of side beam transducerassemblies operated as indicated in FIG. 9, is shown in FIG. 12 of thedrawings. This includes a conventional pipe section 50 in which thereare drilled three openings in a triangular arrangement into which areinterposed three transducer assemblies 51, 52 and 53, of the form shownand described in FIGS. 1 and 2 of the drawings, and operated as shown inFIG. 9. These are interposed in a direction normal to the pipe wall sothat the radiating surface 2b of each of the transducer assemblies issubstantially flush with the inner surface of pipe 50. Transducerassembly 51, which is located at the apex of the triangle, is connectedto the output of the pulse transmitting circuit 49. The two transducerassemblies 52 and 53, which are located at the base angles of thetriangle, are respectively connected to the input terminals of receivers54 and 55, the latter being respectively tuned to two different highfrequency channels designated as No. l and No. 2. The outputs of thesetwo receiver channels are modulated with gating pulses derived from thegate control circuit 56 which, in turn, is synchronized with theoperation of the transmitter circuit 49. The two output channels fromreceivers 54 and 55 are imposed on the divider circuit 57 to provide adifference signal which actuates the flowmeter indicator 58. Details ofthe operation of this circuit may be better understood 'from a study ofUS. Pat. No. 3,204,456, issued Sept.

7, I965, to W. Welkowitz, entitled Ultrasonic Flowmeter.

it will be seen that the side beam transducer assembly 51 whichsimultaneously directs two beam with an angle of between them from theradiating surface thereof to the receiving surfaces of transducerassemblies 52 and 53, provides a novel and advantageous system ofoperation.

Although the patterns disclosed in FIGS. 8 through 12 have beendescribed in general terms, it will be understood that these specificpatterns, as shown, are based on measurements made with air as the fluidmedium.

It will be understood that the invention is not limited to the specificforms of transducer assemblies shown 14 and described herein by way ofexample; or to the specific flowmeter circuit arrangements herein shownand described, but is limited only by the scope of the claimshereinafterset forth.

What is claimed is:

l. A transducer assembly comprising in combination:

an ultrasonic generator which includes a pair of piezoelectric ceramicwafers having substantially parallel opposite major surfaces comprisingelectrode means interposed between the internal major surfaces of saidwafers and on the two external surfaces thereof, said generator designedto vibrate in the direction of the longitudinal axis of said transducerassembly in a principal resonant-thickness mode of vibration.

a cylindrical backing element having a pair of substantially flat endsurfaces,

means interposed between one said end surface of said cylindricalbacking element and one of the opposite surfaces of said generator forbonding said backing element to said one face,

a solid transforming member formed to include at its opposite ends apair of substantially circular parallel disks coaxial with saidultrasonic generator each having a diameter slightly exceeding thediameter of said generator, said disks spaced apart along the axisthereof by a connecting portion symmetrical about the said axis anddeclining therebetween to a minimum diameter not exceeding aboutonethird the diameter of said disks,

one of said disks being bonded in acoustical transfer relation to theother face of said generator, and the other said disk being free tofunction as an ultrasonic radiator.

2. The combination in accordance with claim I wherein the flat majorsurfaces of said cylindrical backing element and of said solidtransforming member each have engraved thereon a series of concentricgrooves,

and said bonding means includes an epoxy cement interposed between saidgrooves in the one face of said backing element and one of the outersurface of said piezoelectric generator,

and means including an epoxy cement interposed between said grooves inthe major face of said transforming member and the outer surface of saidpiezoelectric generator.

3. The combination in accordance with claim 2 wherein said electrodemeans interposed between the major surfaces of said generator wafers andon the external surfaces thereof comprises electrically conductingepoxy.

4. The combination in accordance with claim 2 wherein the axial lengthof said transducer assembly from a plane bisecting the thickness of saidradiator plate to the other end surface of said backing elementapproximates an integral number of quarterwavelengths in the principalvibrating frequency of said transducer, and wherein the diameter of saiddisks approximates a half-wavelength in said vibrating frequency.

5. The combination in accordance with claim 2 wherein the ratio betweenthe mass of said backing element and the mass of said transformingmember including said radiator plate is at least about l:l.

6. The combination in accordance with claim 1 wherein said transducerassembly is enclosed in a tubular inner housing, the end of the cavityof said housing containing the backing element of said asembly beingpartially closed by a plug having a central bore for accommodating apair of energizing leads connected across terminals on the electrodemeans of said ultrasonic generator,

said end plug terminating in an annular flange, an externallyscrew-threaded hollow cylindrical outer housing having an enlargedcylindrical inner chamber constructed to accommodate the flanged end ofsaid tubular inner housing, means comprising a series of o-rings ofelastomer material constructed to fit between said tubular inner housingand the inner chamber of said outer housing for acoustically isolatingsaid innner housing from said outer housing, said outer housing having acoaxially disposed extension, an electrical coaxial connector containinga solid insulating shell between a pair of conductors comprising acentral conductor and an outer coaxial shell accommodated in theextension of said housing, said outer shell being in electrical contactwith the said external housing,

means for connecting one of the electrode terminals of said ultrasonicgenerator to the said central conductor,

and means for providing an electrical conducting path between the otherelectrode terminal of said ultrasonic generator and said externalhousing.

7. the combination in accordance with claim 6 wherein said means forproviding an electrical conducting path between the other electrodeterminal of said ultrasonic generator and said external housingcomprises a conducting strap interposed therebetween.

8. The combination in accordance with claim 6 wherein said means forproviding an electrical conducting path between the other electrodeterminal of said ultrasonic generator and said external housing includesconducting material interposed into said o-rings.

1. A transducer assembly comprising in combination: an ultrasonicgenerator which includes a pair of piezoelectric ceramic wafers havingsubstantially parallel opposite major surfaces comprising electrodemeans interposed between the internal major surfaces of said wafers andon the two external surfaces thereof, said generator designed to vibratein the direction of the longitudinal axis of said transducer assembly ina principal resonant-thickness mode of vibration. a cylindrical backingelement having a pair of substantially flat end surfaces, meansinterposed between one said end surface of said cylindrical backingelement and one of the opposite surfaces of said generator for bondingsaid backing element to said one face, a solid transforming memberformed to include at its opposite ends a pair of substantially circularparallel disks coaxial with said ultrasonic generator each having adiameter slightly exceeding the diameter of said generator, said disksspaced apart along the axis thereof by a connecting portion symmetricalabout the said axis and declining therebetween to a minimum diameter notexceeding about one-third the diameter of said disks, one of said disksbeing bonded in acoustical transfer relation to the other face of saidgenerator, and the other said disk being free to function as anultrasonic radiator.
 2. The combination in accordance with claim 1wherein the flat major surfaces of said cylindrical backing element andof said solid transforming member each have engraved thereon a series ofconcentric grooves, and said bonding means includes an epoxy cementinterposed between said grooves in the one face of said backing elementand one of the outer surface of said piezoelectric generator, and meansincluding an epoxy cement interposed between said grooves in the majorface of said transforming member and the outer surface of saidpiezoelectric generator.
 3. The combination in accordance with claim 2wherein said electrode means interposed between the major surfaces ofsaid generator wafers and on the external surfaces thereof compriseselectrically conducting epoxy.
 4. The combination in accordance withclaim 2 wherein the axial length of said transducer assembly from aplane bisecting the thickness of said radiator plate to the other endsurface of said backing element approximates an integral number ofquarter-wavelengths in the principal vibrating frequency of saidtransducer, and wherein the diameter of said disks approximates ahalf-wavelength in said vibrating frequency.
 5. The combination inaccordance with claim 2 wherein the ratio between the mass of saidbacking element and the mass of said transforming member including saidradiator plate is at least about 1:1.
 6. The combination in accordancewith claim 1 wherein said transducer assembly is enclosed in a tubularinner housing, the end of the cavity of said housing containing thebacking element of said asembly being partially closed by a plug havinga central bore for accommodating a pair of energizing leads Connectedacross terminals on the electrode means of said ultrasonic generator,said end plug terminating in an annular flange, an externallyscrew-threaded hollow cylindrical outer housing having an enlargedcylindrical inner chamber constructed to accommodate the flanged end ofsaid tubular inner housing, means comprising a series of o-rings ofelastomer material constructed to fit between said tubular inner housingand the inner chamber of said outer housing for acoustically isolatingsaid innner housing from said outer housing, said outer housing having acoaxially disposed extension, an electrical coaxial connector containinga solid insulating shell between a pair of conductors comprising acentral conductor and an outer coaxial shell accommodated in theextension of said housing, said outer shell being in electrical contactwith the said external housing, means for connecting one of theelectrode terminals of said ultrasonic generator to the said centralconductor, and means for providing an electrical conducting path betweenthe other electrode terminal of said ultrasonic generator and saidexternal housing.
 7. the combination in accordance with claim 6 whereinsaid means for providing an electrical conducting path between the otherelectrode terminal of said ultrasonic generator and said externalhousing comprises a conducting strap interposed therebetween.
 8. Thecombination in accordance with claim 6 wherein said means for providingan electrical conducting path between the other electrode terminal ofsaid ultrasonic generator and said external housing includes conductingmaterial interposed into said o-rings.